UV is stretching without a reason
I don't know exactly but I have some ideas. I tried to recreate the problem and couldn't, but I created other problems and here's how I fixed them.


If I have a non-manifold mesh (meaning the bottom of the fretboard doesn't have a face, just the top and sides) and I unwrapped, it would unwrap the mesh into 2 pieces... even though I just added edge seams in the same places you did, along the 4 curved "short" edges. And, I got stretching for no reason. https://i.imgur.com/GJlvNxP.jpg
Just as a guess I tried recalculate normals, and it shows the stretching is worse?? https://i.imgur.com/ElknAqI.jpg
So I decided maybe the missing face is the problem. I added a face to the bottom, and added edge seams to 3 out of 4 edges so that everything would stay attached in the UV editor as 1 piece. The stretching is gone, but it's still two pieces, wtf? https://i.imgur.com/PpWyCpE.jpg
Next I figured the problem must be the seams, so I cleared them and carefully added them again in the same places. Almost correct, but 1 small piece is still detached, and it does NOT have seams on all 4 sides. ... hrm. https://i.imgur.com/kU9N4sU.jpg
Finally, I figure out there's an extra vertex somewhere that I didn't want. I did mesh - clean up - remove doubles. This removed the 1 vertex. When I unwrapped again, it's perfect. https://i.imgur.com/92dwf8e.jpg


I think a hidden extra vertex can create this stretch problem... the mesh is attached to it, but you don't see it, because it overlaps some other vertex that it separated from without problems, due to the edge seams you created. 

So my advice is... make sure the mesh doesn't have missing faces. Clean up any extra points or edges or doubles. Recalculate normals. Confirm cuts are in the correct place, then unwrap again. Hopefully one of those ideas fixes it.